In the 10 years since Texas executed Cameron Todd Willingham after convicting him on charges of setting his house on fire and murdering his three young daughters, family members and death penalty opponents have argued that he was innocent. Now newly discovered evidence suggests that the prosecutor in the case may have concealed a deal with a jailhouse informant whose testimony was a key part of the execution decision.

The battle to clear Willingham’s name has symbolic value because it may offer evidence that an innocent man was executed, something opponents of the death penalty believe happens more than occasionally. By contrast, Justice Antonin Scalia wrote seven years ago that he was unaware of “a single case - not one - in which it is clear that a person was executed for a crime he did not commit.”

Willingham was convicted on charges of setting the 1991 fire in Corsicana, Texas, that killed his three children, and was sentenced to death the next year. The conviction rested on two pillars of evidence: analysis by arson investigators and the testimony of a jailhouse informant, Johnny Webb, who said that Willingham had confessed the crime to him.

The arson investigation has since been discredited; serious questions were raised about the quality of the scientific analysis and testimony, which did not measure up to the standard of science even at the time. But the prosecutor who led the case shortly before Willingham’s execution argued that even though the arson analysis had been questioned, the testimony of Webb should be enough to deny any attempt for clemency.

In recent weeks, as part of an effort to obtain a posthumous exoneration from the Texas Board of Pardons and Paroles and Gov. Rick Perry, lawyers working on Willingham’s behalf say they have found evidence that Webb gave his testimony in return for a reduced prison sentence. Evidence of an undisclosed deal could have proved exculpatory during Willingham’s trial or figured in subsequent appeals, but Webb and the prosecutor at trial, John Jackson - who would later become a judge - explicitly denied that any deal existed during Webb’s testimony.

In September, lawyers from the Innocence Project in New York filed an official request with the board to exonerate Willingham, citing the flawed fire science and Jackson’s subsequent actions in the Webb case: efforts to cut Webb’s prison time and to downgrade the charges after the Willingham trial. The Innocence Project also contends that prosecutors suppressed an effort by Webb to recant his testimony.

But recantations in criminal cases are relatively common, said Walter M. Reaves Jr., a criminal defense lawyer who has worked on Willingham’s case, so the biggest open question has been whether Jackson and Webb had made a deal. Judge Jackson, who has retired from the bench, continued, including in an interview last year, to insist there was no deal.

What has changed is that investigators for the Innocence Project have discovered a curt handwritten note in Webb’s file in the district attorney’s office in Corsicana. The current district attorney, R. Lowell Thompson, made the files available to the Innocence Project lawyers, and in late November one of the lawyers, Bryce Benjet, received a box of photocopies.

As he worked through the stack of papers, he saw a note scrawled on the inside of the district attorney’s file folder stating that Webb’s charges were to be listed as robbery in the second degree, not the heavier first-degree robbery charge he had originally been convicted on, “based on coop in Willingham.”

Benjet recalled a “rush of excitement,” he said, and thought, “This is what we’ve been looking for.”

The Innocence Project submitted the note, which is not dated or signed, in a new filing to the board, asking that it be included as part of its September request for a pardon.

Barry Scheck, co-founder of the Innocence Project, called the note a “smoking pistol” in the case.

“We’re reaching out to the principals to see if there is an innocent explanation for this,” he said. “I don’t see one.”

Jackson did not respond to several requests for comment.

Thompson, the district attorney, said that, while he willingly complied with the request for the Webb files, he had no opinion as to what happened during the Willingham trial in 1992. “I wasn’t even a college graduate yet,” he said.

As for Webb, Thompson said, the robbery that put him in a cell with Willingham was not his only brush with the law. “I’ve also prosecuted him,” he said. “The D.A. before me prosecuted him, and the D.A. before him prosecuted him.”

Scheck said that the Willingham case suggested a fundamental weakness in the justice system: If Webb’s testimony “was really based on a deal and misrepresentation, then the system cannot be regulated,” he said. Under those circumstances, “you cannot prevent the execution of an innocent person.”

Even if the board ultimately agrees with Willingham’s advocates, the final decision will rest with Perry (who has called Willingham “a monster” who killed his children) or with his successor in 2015.

Willingham’s stepmother, Eugenia Willingham, said: “I’m real thrilled that all this has come to light. We’ll see what happens. I can’t help but be hopeful.”

His cousin Patricia Cox said that if an exoneration does occur, the family has no plans to press for damages. “We’re not asking compensation,” she said. “We’re asking justice.”

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